Whenever, a Koala Joey reaches the age of 9 months, they have to leave their mothers and onwards, their mothers do become very aggressive towards them, so that the young Koala Joeys make a good life of their own. It is very common within the nature that animals after sometimes abandon their kids and let them to live on their own and do not meet again once they are gone. Therefore, family pact within the Koalas is just a short term process and the same is true about many other animals as well. Mothers do become aggressive towards youngsters, whenever, the youngsters grow up and they have to make a living on their own.
However, by nature Koala mothers are very humble and loving mothers for the young Koala Joeys. They feed them properly, and care them maximum, to grow young Joeys. Even they play with their kids as well when they are about 4 to 6 months old, which also shows a very delicate interaction in between the mother Koala and the Koala Joeys.
Finally, since Koalas do not live as family, they have to leave their mothers. Koalas like many animals in nature live a solitary life. Solitary animals have to live a lonely life and have to meet their mate for a few weeks during the mating season. The same phenomenon becomes valid for the koalas as well. Usually because of their solitary nature, they just select a particular territory of trees (mostly one tree) and make a living on the Eucalyptus leaves.